Engineers at the University of California San Diego have shaken the tallest steel-framed building ever tested on an earthquake simulator. The ten-storey, 30-metre structure was subjected to simulations of real earthquakes, including the 6.9-magnitude, 1989 Loma Prieta temblor, as part of an effort to determine if height limits for buildings made of cold-formed steel could be increased.
The earthquake simulator, or shake table, at the University of California San Diego is one of the three largest in the world and the only one located outdoors. The outdoor location is especially crucial for tests such as these, which push the boundaries in height limits for buildings. Indeed, the shake table is the only place in the world where buildings more than 27 metres tall can be tested. Two years ago, researchers shook a 35-metre mass timber building on the UC San Diego shake table – the tallest building ever tested on a seismic simulator.
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This year’s tests focus on a building made of cold-formed steel (CFS), which is a lightweight, sustainable material that isn’t combustible and is made from 60 per cent to 70 per cent recycled metal. Currently, building codes limit this type of construction to 20 metres, or six storeys. Researchers are asking whether the limit could be increased to ten storeys, or 30 metres, including in seismically active areas. Test results so far point to ‘yes’.
‘The building performed very well,’ said Tara Hutchinson, the project’s lead and a professor in the UC San Diego Department of Structural Engineering. ‘Despite 18 earthquake tests of increasing intensity – including three very large at and above what design engineers must consider in designing a building – the load-bearing structural system retained its integrity.’
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But researchers were expecting some damage to the building’s non-structural components. The stairs, crucial to allowing building occupants to safely evacuate the building and designed to move with the building, were still functional.
‘Within this building, we installed nearly 1,000 sensors to measure its response in terms of acceleration, displacement and local strains – we have an outstanding set of data to analyse and digest, and ultimately to help improve building codes and support the design communities’ desire to use this excellent material in the construction of taller, lightweight, more resilient buildings,’ Hutchinson said.
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Since it’s a lightweight material, CFS can be assembled in modular units, which can then be assembled into a full building, like a giant version of Lego. This technique dramatically reduces the amount of construction time compared to constructing the building’s skeleton from scratch.
‘CFS has a lot of really great features that are going to benefit resilient communities in the future,’ Hutchinson said.
The tests also pointed out the importance of a major upgrade to the shake table. The US$17million project, completed in April 2022, gave the table the ability to move in six degrees of freedom. Before the upgrade, the table could only move in one direction, east to west. Now, it can also move up and down, north to south, and in roll, pitch and yaw motions. For example, in this series of tests, the team subjected the building to 1D, 2D and 3D motions using the same earthquake record, by just turning off different degrees of freedom.
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Recordings from past earthquakes show that the ground doesn’t shake in one direction – it moves back and forth, up and down, side to side and can even wobble, said Joel Conte, one of the lead researchers at the shake table and a professor in the UC San Diego Department of Structural Engineering. ‘Here we are able to simulate what we call near-real world earthquake conditions,’ he said. During a test on 23 June, researchers noticed a certain amount of twisting motion in the building’s movements – something that wouldn’t have happened when the table was only able to move in one direction.
‘The motions that we saw today demonstrated why that table upgrade was critical to the science that we do here,’ said Ben Schafer, an engineering professor at Johns Hopkins University.
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But the test series isn’t yet done. In addition to carefully reviewing the physical state of the building after the earthquake tests, the research team is preparing for a final phase of live-fire testing, which takes place this month. These fire tests are being led by Professor Richard Emberley at Cal Poly-San Luis Obispo, and are aimed at understanding the temperature, smoke and particulate spread throughout compartments in the building that were seismically damaged – a real scenario referred to as ‘fire-following earthquakes’. These could be triggered due to gas or other hazardous substances serving as an ignition source. ‘CFS is non-combustible, unlike wood and some other building materials, an important beneficial characteristic if fires are a concern,’ Hutchinson said.